1. Field of the Invention
The present invention relates to a control and monitor signal transmission system. More specifically, the present invention relates to a control and monitor signal transmission system that transmits a control signal from a controller to a plurality of controllable units via data signal transmission lines that are shared by the plurality of controllable units, wherein a duty ratio of a negative voltage-level period relative to a neutral electric potential of a pulse and a duty ratio of a positive voltage-level period relative to a neutral electric potential of a subsequent pulse thereto are respectively changed according to a value of each data of a control data signal that is connected to the controller, thus providing reliability against noise in communication.
2. Description of Related Art
Securing accuracy of a transmitted signal is one of the most important matters in transmitting a control signal. Another important matter in transmitting a control signal is to reduce the number of transmission lines to perform control signal transmission. Simplifying wiring will reduce wiring space, wiring man-hours, the time required for fabricating an apparatus, and the size of equipment, thereby improving reliability of the equipment and reducing costs.
Patent Documents 1 and 2 disclose examples of a method of reducing the number of electric signal wires in conventional automation and the like. Patent Document 1 describes a method for detecting a control signal in a control and monitor signal transmission system as the presence or absence of a current signal from a sensor to a controller by binarizing transmission of a control signal from a controller to a controllable unit as a predetermined duty cycle ratio. In this instance, the control signal and a monitor signal can be superimposed on a clock signal and therefore a separate electric power line is not required. Consequently, power supply wiring can be simplified. In addition, in this instance, transmission capacity can be doubled since transmission and reception of a signal between the controller and the controllable unit and sensor can be performed bi-directionally and simultaneously.
Patent Document 2 describes a control and monitor signal transmission system having a timing generator in a master station for generating a constant timing signal that synchronizes with a clock signal of a constant cycle, wherein the control and monitor signal transmission system changes a duty cycle ratio of a period of a power supply voltage level of other than the preset power supply voltage value and a period of a subsequent power supply voltage level thereto, relative to a signal value of a control data signal that is transmitted from a controller for each clock cycle under the control of the timing signal, and outputs the control data signal to a data signal transmission line as a serial pulsing voltage signal.
Patent Document 2 also describes a sensor signal reception in which a value of each data of a control data signal is extracted by identifying a duty cycle ratio of a serial pulsing voltage signal for each clock cycle, by a control method using a timing signal; and a slave station input part produces a monitor data signal that is configured of binarized levels of different current signals by timing-signal based control, according to a value of a corresponding sensor, and superimposes the monitor data signal to a predetermined position of the serial pulsing voltage signal, as a data value of the monitor signal. Further, Patent Document 2 describes a control and monitor signal transmission system of the present invention in which a control signal from a controller to a controllable unit is made to be a binarized signal of a predetermined duty cycle ratio; a monitor signal from a sensor to the controller is detected at a rise of power supply voltage based on the presence or absence of a current signal that is generated in contention between the monitor signal and the power supply voltage; and the control signal and the monitor signal are superimposed on a clock signal.
[Patent Document 1] Japanese Patent Laid-open Publication No. 2003-199178
[Patent Document 2] Japanese Patent Laid-open Publication No. 2002-271878
As described above, the methods described in Patent Documents 1 and 2 allow simplification of electric power wiring. Further, the methods allow a fast-speed transmission rate in addition to the simplification of wiring (transmission line), by changing a duty ratio of a signal and performing a signal transmission and reception.
However, when power supply voltage drops, problems occur such as that a signal level also proportionally drops or that threshold voltage is not sufficiently obtained, causing unstable operation and lowering data reliability.
For example, when a battery-operated power supply is used, ambient temperature in a usage environment could affect an electromotive force of the batteries, leading to a considerable drop of output voltage of the power supply. In other words, low ambient temperature decreases battery power output, causing a considerable drop of voltage due to load. Accordingly, it is difficult to maintain a signal level in communication control that uses such a power supply, and a malfunction could occur.
Further, extension of a data signal transmission line is rigidly limited because problems are easily caused, such as a drop of a signal level due to the extension of the data signal transmission line or picking up of noise that is coupled into the data signal transmission line. Meanwhile, there are unavoidable situations in a practical setting, such as that control from a remote location is required in factory premises, that each equipment is scattered throughout remote locations, or that equipment that could interfere with signals is placed in a signal path.